Unknown is the effect of hypothermia or various cardioplegic solution compositions at various temperatures on interstitial myocardial pH during global ischemia and reperfusion. Hydrogen ion flux is very rapid in ischemic tissue and especially so during the initial 30-90 seconds of reperfusion. An NIH developed fiberoptic system using membrane captured phenol red can measure pH in tissues continously. These studies were designed to test the hypothesis that the exponential rate of rise of hydrogen ion concentration in ischemic myocardium is altered substantially by hypothermia because of known pKa changes in water as a function of temperature. Further, the imidazole component of red blood cell buffer is known to interact but is eliminated in these studies by frequent washout of the coronary system. The data show a marked blunting of the time - pH decay with hypothermic and cardioplegic solutions. No significant difference was demonstrated in recovery of functiion between control (pH 7.4) and pH 7.0 groups. Recovery was significantly depressed at terminal ischemia pH values of 6.8 and 6.6. Recovery values were equal at all pH values, although hypothermia at 25 degrees C provides a 2-3 fold increase in time to reach a specific pH value. Cardioplegic solution infusion resulted in little time improvement unless under high pressure and very cold. Prolongation to 92 min was achieved with a single infusion of 1\degree C solution to reach a pH value of 6.8. The system has been used on five patients.